Thermoplastic polyester resin compositions and blends with thermoplastic polyester elastomers are widely used because of their excellent properties. Polyester resin compositions and mixtures thereof with a polyester elastomer are described in European Patent Application No. 0231776. It is disclosed therein that very desirable improvements in impact resistance, heat resistance, cold resistance and weather resistance are achieved if an additive comprising a silicone-based graft copolymer is used. In particular, a modifier comprising a silicone rubber is used onto which a styrene/acrylonitrile comonomer is grafted in the presence of a graft-linking agent. However, such modifiers have relatively poor rubber integrity and incompatibility with the resins, and this leads to delamination in molded articles. Also relevant is BASF, U.K. Patent Specification No. 1,590,549, especially Example 5, which describes a composition comprising 90 wt. percent of poly(butylene terephalate) and 10 wt. percent of a polyorganosiloxane graft polymer impact modifier. Molded articles made from such a composition will, like those of the EPO application above-mentioned, have numerous drawbacks, the most serious of which have been mentioned above, and they will also have a mottled surface appearance.
A novel polyorganosiloxane/polyvinyl-based graft polymer has now been discovered to be extremely useful as a modifier for such thermoplastic resins. It does not show any tendency to cause delamination, while at the same time providing remarkable improvements in tensile properties and impact performance and a reduction in surface gloss.
In the present invention, the polyorganosiloxane rubber substrate of the prior art is replaced by a co-homopolymerized network(s) of polyorganosiloxane/polyvinyl-based polymer(s) in a co-homopolymerization process. Polyorganosiloxane/polystyrene-based co-homopolymers are particularly preferred for use in the first stage of the graft polymers used in the present invention. The present invention uses graft polymers provided by subsequent graft polymerization of vinyl monomers (e.g., polymethyl (meth)acrylate, polystyrene or styrene/acrylonitrile copolymer) onto the polyorganosiloxane/polyvinyl-based polymer product as described above.
This multi-stage graft polymer is usefully employed in the present invention as a superior impact strength modifier for thermoplastic molding compositions comprising polyesters and mixtures thereof with poly(etherester) elastomers, poly(etherimide ester) elastomers or a mixture of the foregoing. They maintain many unique features of silicone rubber such as enhanced impact strength, particularly at low temperature, while maintaining other desirable properties.